Structural materials today need to operate at higher temperatures. Most ceramic materials have good long-term stability against creep and chemical attack at temperatures above the operating range for current alloys. Because of ceramic's low fracture energies, however, ceramics are subject to catastrophic failure. Even relatively small defects can start propagation of cracks that can catastrophically propagate through the ceramic component. Therefore, measures for improving their fracture toughness, i.e., toughening ceramics without sacrificing their excellent properties, are sought after.
Fiber-reinforced ceramic composites are being considered as the next generation of high temperature structural materials for advanced aircraft engines and gas turbines. They possess higher temperature capability and lighter weight than those of the currently used superalloys. In these potential applications, fiber-reinforced ceramic composites are subjected to severe thermal and mechanical conditions. Although the fiber-reinforced composites are designed to be used below their matrix cracking stress, accidental overstressing, either thermally as a result of a thermal shock or mechanically during a foreign object impact, can hardly be avoided.
Cracks will be generated in the fiber-reinforced composite matrix when the composite is subjected to a higher stress than its matrix cracking stress. Such cracks will stay open even if the operating stress is subsequently reduced to a value below the matrix cracking stress, exposing coatings and/or fibers to the environment. As a result, the existence of cracks in fiber-reinforced composite matrices will affect the performance and durability of the composites, especially if the cracks are through the thickness of the composites.
These cracks can serve as a fast path for the transport of environmental gaseous phases into the composite. Oxygen can diffuse very rapidly through even extremely small cracks in the matrix. The fibers and any coating that may be on the fiber can oxidize by oxygen diffusing through the crack. Oxygen reacts with the fiber coating and eventually the fiber, causing local bonding between the fiber and matrix. Fiber failure will initiate at this bonded location because of the resultant stress concentration and fiber degradation. This process continues until the remaining fibers are unable to carry the load, and the composite fails at a stress appreciably less than the ultimate strength. The composite also loses its tough behavior because of the strong bonding between the fiber and the matrix. Thus, a serious problem limiting the life of ceramic composites is the oxidation of the fiber coating followed by oxidation of the fiber at the base of the crack.
The ceramic composites of interest for engine applications have focused on carbon-carbon composites, having a carbon matrix with carbon fibers, and silicon carbide composites, which have a silicon carbide matrix with silicon carbide fibers, the fibers usually being coated. An important limitation to the use of carbonized structural materials is their susceptibility to oxidation in high temperature, oxidizing environments. Oxygen attacks the surface of the carbonized material and seeps into the pores of interstices that invariably are present, oxidizing the surfaces of the pores and continuously weakening the material. The oxidizing atmosphere reaching the fibers, carbon and graphite fibers, seriously degrades the composite structure.
An approach to overcome the oxidation of carbon-carbon composites has been the use of glass-formers as oxidation inhibitors. The glass-formers are used as coatings surrounding the carbon matrix, and/or placed between carbon fiber plies. The patents that teach the use of glass-formers as oxidation inhibitors in carbon-carbon composites are U.S. Pat. No. 4,795,677; U.S. Pat. No. 4,894,286; U.S. Pat. No. 4,892,790; and U.S. Pat. No. 4,599,256.
In spite of the advances that have been made in carbon-carbon composites, there is still a demand for improved ceramic composites with higher temperature and mechanical capability. Silicon carbide and silicon carbide-silicon matrix composites are currently of interest. These composites can be made by various methods. One method of making silicon carbide composites is the use of chemical vapor infiltration. Here, layers of cloth made of the fiber material are coated with boron nitride by chemical vapor infiltration. This takes about one day to deposit about 0.5 micrometers of boron nitride. The layers of cloth are then coated with silicon carbide by chemical vapor deposition for about 10 to 20 days. An approach to overcome the oxidation of silicon carbide composites has been the use of an oxygen-scavenging sealant-forming region in intimate contact with the ceramic fibers and a debonding layer, which is in intimate contact with the ceramic fibers, as described in U.S. Pat. No. 5,094,901.
A method of making silicon carbide-silicon matrix composites reinforced with silicon carbide-containing fibers is by using molten silicon melt infiltration into a preform. In this process, silicon carbide fibers are bundled in tows and coated with a coating or combination of coatings selected from the group consisting of boron nitride, pyrolyzed carbon, silicon nitride, carbon, and mixtures thereof. In one version of silicon carbide-silicon composites, the coatings comprise layers of boron nitride and silicon carbide or silicon nitride. The tows are laminated to make a structure, which is then infiltrated with molten silicon. In these methods a boron nitride coating on the fiber is used to protect the fiber from attack by molten silicon or for debonding. Another method used to make silicon carbide-silicon composites uses fibers in the form of cloth or 3-D structure, which are layered into the desired shape. Boron nitride coating is deposited on the cloth layers by chemical vapor infiltration as mentioned above, and silicon carbide coating is deposited also by chemical vapor infiltration for about 5 days to achieve a thickness of about 2 micrometers. The structure is then processed in a slurry and melt infiltrated with molten silicon. The molten silicon may contain minute amounts of boron.
Recently, reinforcing silicon carbide-silicon matrix composites with strong silicon carbide fibers have been shown to increase their fracture energy substantially and exhibit tough failure mode. The increased fracture toughness of silicon-silicon carbide matrix composites, combined with their high creep resistance and chemical stability at high temperatures, make them promising materials for use as structural components in hot sections of heat engines and gas turbines.
When silicon carbide-silicon matrix composites develop fine cracks as a result of loading beyond the first matrix cracking stress, the silicon carbide fiber as well as the crack surface is exposed to oxidative environments. This can occur in dry oxidative environments as well as wet or water vapor-containing environments, such as encountered under humid conditions in combustion engines where there are combustion gaseous products of fuels. The oxidative attack is rapid at high temperatures. The oxidation of the crack surface and the fiber may make the composite brittle. The result may be a weaker composite and premature failure of a component part made from the silicon carbide-silicon composite.
Thus, there is a need for a ceramic composite that successfully seals cracks in silicon carbide-silicon matrix composites to prevent the ready access of oxygen at high temperatures, above about 600.degree. C. There is also a need for a method to make silicon carbide-silicon matrix composites and articles made from molten silicon infiltration that heal matrix cracks in dry and water vapor-containing environments at high temperatures, greater than about 600.degree. C.